This invention relates generally to an automatic collision avoidance control and alert system for tracking and directing aircraft and other vehicles for collision avoidance. More specifically, the invention is an improved collision avoidance system for use in automatically alerting a pilot of a collision threat and coordinating an evasive maneuver between aircraft. The expected advantages of such a system is improved collision avoidance, elimination of false collision alarms and an increased target tracking capacity. The present invention is directed to such an automatic vehicular-based system for automatically providing collision avoidance.
Since the advent of aviation it has been desirable to avoid aircraft collisions and near misses. Traditionally, pilots have used a wholly manual method, i.e. visually identifying other aircraft and flying to avoid a collision. Such a system is susceptible to human error and is wholly unworkable in low visibility conditions or within crowded airspace.
The use of a ground based air traffic control (ATC) system, two-way voice radio communications and RADAR greatly enhance the identification and control of aircraft to avoid collisions. Two-way voice radio communication allows aircraft to communicate with one another and the ATC operator to avoid potential conflicts. RADAR, both on-board aircraft and at ground based ATC facilities provides an operator intensive technique for avoiding aircraft collisions. In the present ATC system the ATC operator coordinates the location, altitude, and track of all aircraft within her assigned control area by communicating with the aircraft over two-way voice radio. Present ATC systems consist of a network of airport terminal area and enroute surveillance radar systems. These systems consist of both primary and secondary RADAR systems and computers that display usable data for the control of air traffic in the national and international airspace systems.
The basic ATC RADAR system consists of Primary RADAR and secondary RADAR. Primary RADAR operates by transmitting a high power, highly directional radio pulse at a known azimuth (direction, in degrees from North) from a rotating antenna and measures the time it takes to receive the reflected signal from an object (aircraft) in space back to the point of transmission. This time factor determines the range in nautical miles from the radar site to the target. The direction of the target is determined by the antenna azimuth from which the signal is received. The limitations of using only this system result in the loss of targets because of the difficulty in detecting weak reflected RADAR return signals attenuated by atmospheric conditions and the difficulty in operating a synchronized height finding radar.
Secondary RADAR known as the Air Traffic Control Radar Beacon System (ATCRBS) utilizes cooperative equipment (a radio receiver/transmitter or transponder) located in the target aircraft to replace the conventional radar's passive reflected return signal with an active reply signal. Like a conventional high power radar, ground based secondary radar transmits a highly directional pulse from a rotating antenna that is usually synchronized with the primary radar antenna. The secondary radar pulse is called the interrogating signal. The interrogating signal requires much less power than conventional radar because secondary radar relies on an active return signal from the target aircraft. In response to receiving the interrogating signal the cooperative aircraft transponder automatically transmits a distinctive reply signal back to the secondary radar's antenna. The secondary radar measures the time between the interrogating signal transmission and the transponder reply signal and, like the reflected return in primary radar, uses this time delay to determine the range of the target aircraft. The direction of the target aircraft is determined by the antenna azimuth from which the reply signal is received. The secondary radar's cooperative transponder improves on the conventional radar's passive reflective return by encoding additional information in the transponder reply signal. The additional information includes an aircraft identification number and the aircraft pressure altitude. For example, Delta flight 195 to Dallas (Dall95) is requested by ground based ATC to squawk "4142". In response, the aircraft pilot manually dials in "41420" at the aircraft transponder control panel. The transponder control logic can now encode the assigned four digit identification, e.g. "4142", on the transponder reply signal. The aircraft's transponder can also be connected to the aircraft's pressure altimeter to enable the transponder control logic to encode the aircraft pressure altitude on the transponder reply signal. The aircraft transponder reply signal containing the encoded aircraft identification and pressure altitude is processed by ground based computers for display on the ATC operator's radar screen. The ATC operators usually provide specific flight instructions to aircraft to avoid flight conflicts and warn aircraft of other nearby aircraft. In large aircraft, active on-board conventional nose RADAR may also identify aircraft that are in front of the large aircraft.
RADAR, however, has a number of disadvantages. Radar systems, even secondary radar, provides limited range and accuracy in the determination of the location and altitude of an aircraft. The range of radar is inherently limited due to obstacles in the line of sight of the radar, curvature of the earth, atmospheric conditions, etc., and is subject to provide false readings or ghosts. RADAR may also fail to provide sufficient target resolution at the critical near collision phase where target aircraft are close together. Radar coverage is not available in many areas of the world, and is not available at all altitudes in the United States.
The presently used and Federal Aviation Administration (FAA) approved aircraft collision avoidance system is known as the Traffic Alert and Collision Avoidance System (TCAS). The TCAS is an airborne traffic alert and collision avoidance advisory system that operates without support from ATC ground stations. TCAS detects the presence of nearby intruder aircraft equipped with transponders that reply to secondary radar interrogating signals. TCAS tracks and continuously evaluates the threat potential of these aircraft in relation to one's own aircraft, displays the nearby transponder-equipped aircraft on a traffic advisory display, and during threat situations provides traffic advisory alerts and vertical maneuvering resolution advisories (RA) to assist the pilot in avoiding mid-air collisions. A TCAS has a transmitter, a transmit antenna, a transponder, one or two directional receiver antennae, a control interface, display unit(s), and a signal/control processor.
A TCAS determines the location of other aircraft by using the cooperative secondary radar transponders located in other aircraft. A TCAS transmitter asynchronously polls for other aircraft with an active L-band interrogating signal, i.e. at the same frequency as the ground based secondary radar interrogating signal. The TCAS interrogating signal, however, is an omni-directional signal whereas the ground based secondary radar signal is highly directional. When a target aircraft's cooperative transponder receives a TCAS interrogating signal the transponder transmits a reply signal. By using RADAR timing principals, the interrogating TCAS can measure the time between the interrogating signal transmission and transponder reply to determine the approximate range of the intruder aircraft. By using direction finding antenna techniques, the interrogating TICAS determines the relative direction of the transponder reply signal with a fixed directional antenna array and the TCAS signal processor. The TCAS omni-directional interrogating signal causes all secondary radar cooperative transponders within receiving range to reply, therefore, the TCAS signal processor uses a complex receiver input blanking scheme to locate and distinguish the multiple reply signals. For example, the TCAS interrogating signal is coordinated with the TCAS signal processor to allow the TCAS signal processor to create a variable width receiver blanking signal. The variable width receiver blanking signal is used to progressively exclude "closer" transponder reply signals. This allows the TCAS interrogator signal and transponder reply signal processor to receive transponder replies from progressively further away aircraft.
The TCAS control logic uses the range, relative bearing, and pressure altitude determined by the interrogating signal and secondary radar transponder replies to track intruder aircraft. The intruder track is displayed on the TCAS display. The TCAS display is a VDU typically mounted on the aircraft front instrument panel. TCAS tracking information, graphically depicting the relative distance and relative bearing of intruder aircraft, greatly assists a pilot in identifying and visually acquiring intruder aircraft. The TCAS control logic also calculates the "tau" of the intruder aircraft. "Tau" is the ratio of range to range-rate, and represents the time to intercept for two aircraft on a collision course, assuming un-accelerated relative motion. The TCAS compares the "tau" with pre-determined collision threat parameters. If an intruder aircraft falls within these parameters a TCAS declares the intruder aircraft a threat. The pre-determined collision threat parameters delineate the intruder aircraft "tau" into four threat categories. In most categories the TCAS merely brings the intruder status to the pilot's attention with an audible alert on the aircraft intercom. In the highest category the TCAS creates an evasive maneuver to vector both aircraft to increase the vertical separation between the aircraft. For situations where one's own TCAS equipped aircraft and another TCAS equipped aircraft are declared collision threats to each other, the TCAS in each aircraft in conjunction with their secondary radar transponder subsystem, establish an air-to-air resolution advisory in both aircraft. The resolution advisory is displayed on the TCAS display unit. The resolution advisory is a directive to the pilots to either climb or descend.
The TCAS system, however, suffers from a number of disadvantages. First, the TCAS system issues numerous false alarms and/or erroneous commands or instructions. Erroneous commands and false alarms may increase the probability of collision by erroneously instructing the aircraft to fly nearer the intruding aircraft, or to descend when the aircraft is already at a minimal altitude. Such false alarms and erroneous instructions are prevalent during take-off and landing where the TCAS has particular trouble discerning signals from the many nearby aircraft. Such false alarms, in addition to distracting the pilot, can create distrust in the entire TCAS system. This distrust can cause a pilot to hesitate or ignore a valid evasive maneuver or resolution advisory command because the pilot mistakenly believes the command is just "another" TCAS false alarm. Another source of TCAS false alarms is an overly simplistic collision prediction algorithm, i.e. the "tau" calculation. The TCAS collision alert algorithm does not account for whether an aircraft is proceeding on it's present course or is leveling off at a predetermined altitude, i.e. it assumes an un-accelerated aircraft track. Such problems are reported by Dave Davis and Michael Sangiacomo in Jets in Jeorardy; False Warnings from Midair Collision System have Led Airline Pilots to Near Catastrophe, The Plain Dealer, Jul. 14, 1994, at 1A. The Plain Dealer investigative reporters discovered that within the span of a few months, TCAS false alarms nearly caused several aircraft disasters. Furthermore, some pilots state that the TCAS is so unreliable in crowded airspace that the TCAS does not work at all under these conditions.
Second, TCAS requires an elaborate direction finding antenna array and processing logic to find an intruder's relative direction. Such a system is inherently susceptible to multipath errors, noise clutter, and other spurious signals. Again, this can create false alarms, false returns or phantom aircraft and incorrect tracking displays.
Third, intruder pressure altitude information is only transmitted from the TCAS transponder when the TCAS is connected to a pressure altimeter. Thus, the existing TCAS system cannot detect a collision danger with an aircraft that does not have a functional pressure altimeter. Moreover, the pressure altimeter itself is subject to the risk of human error. A pressure altimeter must be periodically adjusted to compensate for local atmospheric conditions and elevations. If the pilot does not make the proper altimeter adjustment then the TCAS transponder will transmit erroneous altitude information. The lack of altitude information or incorrect altitude information seriously undermines the accuracy of the TCAS system.
Fourth, the TCAS interrogator/transponder protocol requires an elaborate antenna sidelobe suppression and receiver blanking technique to block transponder replies from close aircraft and to allow the system to poll aircraft further away. Such a system is again susceptible to noise, signal clutter, other spurious signals and multiple aircraft transponder within the same transponders reply time frame.
ATC systems have been proposed that would use the global positioning system (GPS) satellites. Such a proposed system is discussed in chapter 12 of Logsdon, The Navstar Global Positioning System, Von Neistrand Reinhold (1992). In The Navstar Global Positioning System, Logsdon discusses the proposed use of GPS receivers on board aircraft, wherein the aircraft transmits its GPS aircraft vector to air traffic controllers for display on the air traffic controllers'to screen. Logsdon also discusses another proposed navigation system based on the proposed Geostar satellites. In the Geostar system, when an aircraft needs to know its location, interrogation pulses are transmitted from the aircraft to three Geostar satellites, which immediately relay the request to a centrally located computer on the ground. The ground computer determines the location of the aircraft and relays the location back to the aircraft using one of the satellites. The Geostar system was described as also being able to relay short telegram messages between two Geostar subscribers (aircraft) using one of the satellites. This proposed Geostar navigation system suffers from a number of drawbacks. For example, as with TCAS, each aircraft does not provide its location, speed, heading to other aircraft. Instead, in Geostar, aircraft must rely on the extensive ground-based processing before the aircraft can obtain its own position. Furthermore, the Geostar system provides no technique for reducing collision of aircraft.
Other navigation aides are know to the art. In the 48 contiguous United States, most instrument navigating is done with the aid of a VHF Omnidirectional Range (VOR) receiver for using the VHF radio signals emitted by the ground based VOR transmitters. Virtually all enroute navigation and many instrument approaches use these signals, which are broadcast in the frequency range 108.0 to 119.0 Mhz. The VOR signal is a blinking omnidirectional pulse, and has two parts: a reference phase signal and the variable phase signal. It is transmitted in such a way that the phase between these two signals is the same as the number of degrees the receiving aircraft is from the VOR station. The VOR receiver and equipment uses the signals to determine the aircraft direction, or course, from the VOR.
An additional navigation aide is known as Distance Measurement Equipment (DME). DME uses two-way (interrogation and reply) active spherical ranging to measure the slant range between the aircraft and the DME transmitting station. Many pilots and navigators vector airplanes from waypoint to waypoint using the signals from VOR/DME, rather than traveling in a straight line. As a result, aircraft are not traveling the shortest distance, causing increased fuel usage and increased travel time. Also, routes along the VOR/DME stations become heavily traveled resulting in increased probability of mid-air collisions.
In addition, many aircraft employ so-called Instrument Landing Systems (ILS) for performing precision landings. ILS includes several VHF localizer transmitters that emit focused VHF signals upwardly from the airport to provide horizontal guidance to the aircraft and its autopilot systems. ILS also includes a UHF glideslope transmitter that radiates a focused UHF signal that angles downwardly across the runway to provide vertical guidance. While ILS provides an effective technique for precision landings, such ILS precision landings are not possible where the airport does not include such localizer and glideslope transmitters.